"My alarm panel is somewhat isolated in the house with no nearby Ethernet connection, so I added an Xbee wireless module capable of sending data to another room in the house with Ethernet available. The Arduino spends its time monitoring the alarm output pins and sending the latest status over the air. Of course an SMS is ultimately the best way to send an urgent message like an intruder alert, but the Twitter updates are useful for posting less important status updates. As well as the intruder alert signal, the module can read a bunch of other status signals from the alarm and post those signals only to Twitter."

I didn’t see any mention of which alarm he was interfacing with in the original article – I suppose given the reading of status signals it’s something more advanced than the default alarm, but would be nice to know which one…